For long Term Evolution (LTE), Remote Radio Unit (RRU) must support high order modulation signal, such as 64 Quadrature Amplitude Modulation (QAM). If filter unit (FU) frequency response (in both phase and amplitude) could not be compensated, EVM link budge become too tighten. EVM characterizes modulation precision, and is a key index of judging digital modulation quality in modern wireless communication system. EVM is vector difference between ideal measurement component I (In Phase) and Q (Quadrature Phase) (referred to as reference signal “R”) of transmitted signal and I, Q component amplitude of the actually received measurement signal “M”.
In prior art, to resolve the problem, Ericsson has imported FU equalizer to WCDMA RU and RRU product in order to improve EVM performance. However, FU must be placed in a factory, then characteristics of FU is to be tested and stored in a database. RRU application software read it from said database, then calculates the matched Finite Impulse Response (FIR) filter to inverse said characteristics of FU, whereby compensating the FU frequency response and improving the EVM performance. It takes a liter bit time for production and adds a small cost.
Problems with prior art are as follows:
1. Each FU needs external test equipment/time;
2. External storage (or data space) is needed to store the FU characteristics;
3. After being delivered to operator, the change of FU, temperature change, aging etc could no be tracked in real time;
4. During the production, the FU could not replace without calibration and external cost for RRU repair on production line are added.
Consequently, there is a need for a method and a device for compensating frequency response of FU in RRU in real time.